1. Field of the Invention
The present invention relates to a method for producing photochromic lenses by laminating an ultra thin lens to a support lens with a photochromic layer therebetween.
2. Description of the Related Art
Lenses and other articles manufactured at optical quality have exacting demands for mold replication, high optical transmission and impact resistance. In the 2002 Polycarbonates publication, Brunelle and Kailasam describe how polycarbonate is prepared by the transesterification of a bisphenol-A with a carbonate. U.S. Pat. No. 5,212,280 describes diphenols which are useful in the condensation method of producing polycarbonate in the presence of phosgene.
Injection molding of lenses requires edge gating so that the runner ends up remote from the lens surfaces. The paths from the gate to the edge points of the mold cavity are not symmetrical and therefore make it difficult to control the thermodynamics of the cooling melt flow. As lens cavities become thinner, straight injection molding techniques are unable to fill the mold without premature freeze-off. Accordingly, injection molding machines have been modified to enlarge the cavity during some phase of the injection cycle, in a so-called injection/compression process. Recent improvements in injection molding techniques by the applicant have resulted in ultra thin lenses that can be effectively incorporated into laminated lenses.
Previously, relatively thick lenses were utilized in laminated optics, as can be seen for example in U.S. Pat. No. 6,256,152. The process as described in all of the examples, uses a pair of 2.5 mm center thickness Diallyl glycol carbonate lenses. Dially glycol carbonate is a thermoset polymer sold under the trade name CR-39, that is formed into lenses by casting. CR-39 is more brittle, and therefore less flexible, than polycarbonate. As a result the prior art requires perfectly matching base curves on the mating surfaces of the two lenses. An inner film or foil is cast, molded or blown into a solid form having the same base curve as the interfacial surfaces of the two lenses. Despite the uniform base curves, the prior art requires an additional step of treating the interfacial surfaces with plasma or corona discharge inter alia, in order to modify or improve bondability.
U.S. Pat. No. 4,867,553 also relates to cast CR-39 lenses having a center thickness at least 1.0 mm and an edge thickness of approximately 1.7 mm. The patent describes a two component assembly. The cover lens can include coatings, filters or tints. However, within the specified ratio of 1.5 to 2 times more edge thickness than center thickness, a photochromic dye will appear much darker in the thicker, peripheral portions than it will in the center.
Accordingly, it would be desirable to provide a lens assembly having a uniformly thick photochromic layer, along with a streamlined process to laminate a thin and flexible front lens thereon, without having to supply both of the lenses and the photochromic film in the same base curve.